The present invention relates in general to wireless communication systems, such as but not limited to wireless local area networks (WLANs), and is particularly directed to a new and improved RAKE receiver that contains an embedded decision feedback equalizer (DFE) that increases the receiver""s tolerance to the effects of (indoor WLAN) multipath distortion without losing robustness to thermal noise.
The ongoing demand for faster (higher data rate) wireless communication products is currently the subject of a number of proposals before the IEEE 802.11 committee that involve the use of a new standard for the 2.4 GHz portion of the spectrum, which FCC Part 15.247 requires be implemented using spread spectrum techniques that enable intra-packet data rates to exceed 10 Mbps Ethernet speeds. The 802.11 standard presently covers only one and two Mpbs data rates, that use either frequency hopping (FH) or direct sequence (DS) spread spectrum (SS) techniques. The FCC requirement for the use of spread spectrum signaling takes advantage of inherent SS properties that make the signals more robust to inadvertence interference by lowering the average transmit power spectral density, and through receiver techniques which exploit spectral redundancy and thereby combat self-interference created by multipath distortion.
As shown in FIG. 1, the power delay profile (PDF) 10 of a transmitted signal due to multipath distortion within an indoor WLAN system, such as the reduced complexity example illustrated in FIG. 2, exhibits a largely exponentially-decayed Rayleigh fading characteristic. Physical aspects of the indoor transmission environment driving this behavior are the relatively large number of reflectors (e.g., walls) within the building, such as shown at nodes 12 and 13, between a transmitter site 14 and a receiver site 15, and the propagation loss associated with the respectively later time-of-arrival propagation paths t1, t2 and t3, which contain logarithmically weaker energies.
The power delay profile of the signal is the variation in mean signal power with respect to its power dispersed across time. The mean power level of the signal establishes the variance of its corresponding Rayleigh components. A principal aspect of the exponentially decayed multipath effect is due to the fact that a signal""s propagation delay t1 is proportional to the total distance traveled, so that, on average, the strongest (minimal obstruction containing) transmission paths are those whose signals are the earliest to arrive at the receiver. In a given stochastic occurrence, a first to arrive, direct or line-of-sight path from the transmitter site 14 to the receiver site 15 may encounter an attenuating medium (such as one or more building walls and the like), while a later arriving signal reflected off a highly reflective surface and encounter no attenuating media may have a larger channel impulse response (CIR) than the first-to-arrive signal. However, on average, such occurrences are few in number relative to the number of echo signals which follow the CIR peak.
In terms of a practical application, the root mean squared (RMS) delay spread of a multipath channel may range from 20-50 nsec for small office and home office (SOHO) environments, 50-100 nsec for commercial environments, and 100-200 nsec for factory environments. For exponentially faded channels, the (exponential) decay constant is equal to the RMS delay spread. For relatively low signal bandwidths (less than 1 MHz), fading due to multipath is mostly xe2x80x9cflatxe2x80x9d. However, at bandwidths above 1 MHz, for example at the 10 MHz bandwidth required by a direct sequence spread spectrum (DSSS) system to attain the above-referenced higher data rate of 10 Mbps, fading becomes selective with frequency, constituting a serious impediment to reliable communications over a multipath channel. Thus, multipath distortion within a WLAN environment can cause severe propagation loss over the ISM band.
A preferred mechanism to counter this severe frequency-selective multipath distortion problem is a channel-matched correlation receiver, commonly referred to as a xe2x80x9cRAKExe2x80x9d receiver. For successful RAKE receiver operation, it is necessary to use a DSSS structure having a transmitted bandwidth larger than the information bandwidth. In a DSSS signal structure, a respective codeword is formed of a sequence of PN code xe2x80x9cchipsxe2x80x9d. The term xe2x80x9ccodewordxe2x80x9d, rather than xe2x80x9csymbolxe2x80x9d, is employed here to avoid confusion between xe2x80x9cchipsxe2x80x9d and codewords. The DSSS chips may be transmitted using a relatively simple modulation scheme such as QPSK, and codeword chips may be fixed as in a signature sequence, or they may be pseudo random.
In addition, phase modulation of the codeword may be used to convey information. Namely, to impart additional bits of information per codeword, the codeword may be shifted in phase. For example two additional bits may be used to provide quadrature (ninety degree) phase shift increments: 0xc2x0, 90xc2x0, 180xc2x0 and 270xc2x0. The codeword""s chips may be selected from a multi-codeword set, where M bits select a particular codeword out of N codewords that make up the multi-codeword set. An example of such a scheme is the use of Walsh or Hadamard codes for the codeword set. For the above-referenced 2.4 GHz spectrum, the IEEE 802.11 standards committee has proposed using an eight bit encoding scheme, in which sit bits select one of N=64 multi-chip codewords, and the remaining two bits define one of four possible (quadrature) phases of the selected codeword.
As diagrammatically illustrated in FIG. 3, in a channel-matched correlation or RAKE receiver, the received (spread) signal is coupled to a codeword correlator 31, the output of which (shown as a sequence of time-of-arrival impulses 32-1, 32-2, 32-3) is applied to a coherent multipath combiner 33. The codeword correlator 31 contains a plurality of correlators each of which is configured to detect a respectively different one of the codewords of the multi-codeword set. The coherent multipath combiner may be readily implemented as a channel matched filter (whose filter taps have been established by means of a training preamble prior to commencement of a data transmission session). The output of the coherent multipath combiner 33 is coupled to a peak or largest value detector 35, which selects the largest output produced by the coherent multipath combiner as the transmitted codeword. Since the RAKE receiver is a linear system, the order of the operations carried out by the channel matched filter (coherent multipath combiner) 33 and codeword correlator 31 may be reversed, as shown in FIG. 4, wherein the channel matched filter 33 is installed upstream of the codeword correlator 31.
A RAKE receiver works reasonably well, since it coherently combines the multipath received signals plus echoes into a single composite signal. By proper choice of the codewords that make up the codeword set, the echoes can be effectively eliminated during codeword correlation. Ideally, each codeword of the set has the following properties: 1xe2x80x94an impulse auto-correlation function; 2xe2x80x94it is mutually orthogonal (has a zero cross-correlation function) to all other codewords of the set; 3xe2x80x94is long relative to the multipath spread; and 4xe2x80x94it has the same energy of each of the other N codewords of the set.
If properties 2 and 4 are absent, the RAKE receiver must establish an orthogonal basis and account for the imbalance, just as in quadrature amplitude modulationxe2x80x94a receiver complexity issue. Also, the codewords need not be long relative to the multipath spread so long as the codewords are impulsive and have zero cross-correlation functions (namely, no intercodeword or intersymbol interference (ISI)). Interchip interference impacts only receiver energy (if the impulsive auto-correlation property is absent). Although an optimal RAKE receiver imparts orthogonality to the codeword correlator output and makes a decision by observing all correlator outputs, no RAKE receiver is ideal, since it is effectively impossible to generate codewords having impulsive auto-correlation functions and zero cross-correlation functions.
In addition, for severe multipath, in order to minimize degradation due to intercodeword interference, the codeword length must be very large (e.g., on the order of 64, 128, 256 or above, as used in military applications). However, in commercial environments, the number of chips per codeword must be limited in order to maximize useable data bandwidth. Since the extent of codeword bleedover increases as the number of chips per codeword is reduced, where multipath distortion is significant, a very small codeword chip density may cause codeword energy bleedover/leakage across multiple codewords. The problem, therefore, is how to optimize the signal-to-noise ratio of the output of the RAKE receiver using such less than ideal codewords.
In accordance with the present invention, this problem is successfully addressed by an enhanced RAKE receiver architecture that contains a chip-based decision feedback equalizer (DFE) structure embedded in the signal processing path through the receiver""s channel matched filter and codeword correlator. This decision feedback equalizer serves to reduce or cancel two types of distortion to which the limited chip length codewords are subjected as they are convolved with the multipath channel during transmission.
The first is the xe2x80x9cbleedingxe2x80x9d or leakage of energy in a respective codeword CW1 with that of another codeword CWi+j. The second form of distortion is a xe2x80x9csmearingxe2x80x9d of the energy within the chips of a respective codeword.
A decision feedback equalizer is especially suited for combating indoor multipath distortion in a WLAN, since this type of multipath distortion is predominantly minimum phase, as the strongest signal components almost always arrive first, while the weaker components arrive last. Therefore, most of the multipath distortion appears as a decaying xe2x80x9ctailxe2x80x9d on the channel impulse response. Moreover, the feedback taps of a DFE serve ideally to combat minimum phase multipath distortion, while the feed-forward taps combat maximum phase components. As a result, a DFE for combating indoor multipath requires very few feed-forward taps, with most of the processing being executed in the feedback taps. Since implementing DFE feed-forward taps at baseband requires full complex multipliers, while feedback taps need only complex addition and subtraction when QPSK elements are employed, implementation complexity is not a significant issue.
Although embedding a DFE into a RAKE receiver readily combats indoor multipath, it makes decisions on a chip, and therefore requires relatively high SNRs. The feedback taps eliminate the long decaying multipath echo tail and can span multiple codewords, so as to cancel inter-chip smearing and inter-codeword bleedover. The codeword correlator improves the signal-to-noise ratio by coherently combining the codeword""s soft decision chips. Should a DFE chip-decision error be made, the codeword correlator is still able to make a correct decision by coherently combining all of the codeword""s chips.
For lower signal-to-noise ratios, error propagation in the decision feedback equalizer causes chip errors to occur in bursts. If soft chip decisions for setting the tap coefficients of the equalizer are incorrect, the DFE tap-weighting characteristic will rapidly degrade, preventing multipath distortion compensation. To handle low SNR, all of the received codeword chips are examined prior to making a hard decision. This is accomplished by generating a DFE-based correlation detection statistic for each codeword that could have been sent. The detection statistic for a potentially transmitted codeword is generated by performing feedback equalization of the codeword""s chips, with the presumption that the codeword was actually transmitted.
A DFE-embedded signal processing architecture for canceling inter-codeword interference may be xe2x80x9cwrapped aroundxe2x80x9d the codeword correlator. In such an architecture, the channel matched filter is differentially combined with a post-cursor representative echo that is produced by estimating the channel impulse response. The result is a xe2x80x9ccleaned-upxe2x80x9d copy of the received codeword, which is coupled to the RAKE receiver""s codeword correlator. The correlator output is coupled to codeword decision operator which examines all of the chips in a received codeword to make a decision as to what codeword was actually transmitted. The codeword decision is used to synthesize a replica of the chip contents and phase information of the transmitted codeword. This synthesized codeword is then convolved with an estimate of the channel impulse response implemented in a FIR filter to produce the representation of the post-cursor multipath echo in the signal received by the channel matched filter.
To cancel intra-codeword chip-smearing of the energy within the chips of a respective codeword, the signal processing branch through each respective codeword correlator is configured to differentially combine the contents of all of the chips that make up each received codeword with a respective one of different sets of codeword-associated DFE feedback taps, that represent the post cursor multipath distortion echoes experienced by that particular codeword in the course of its transmission over the multipath channel from the transmitter. The post cursor multipath distortion may be removed either upstream or downstream of the codeword correlator.
In the upstream implementation, each successively received codeword chip set is coupled to a plurality of codeword correlator statistical branches, each of which is associated with a different codeword. For the non-limiting example of using an eight bit field, there are K=256 codeword combinations (comprised of 26=64 codewords, each QPSK encoded at one of 22=4 possible quadrature phases (0xc2x0, 90xc2x0, 180xc2x0, 270xc2x0)). In a respective codeword correlator branch, the received signal path is differentially combined with the output of an FIR filter feedback tap set, that synthesizes the multipath channel impulse response for its respective codeword""s chip set (e.g., comprised of eight chips per codeword in the present example), so as to produce a representation of the post-cursor multipath echo for that codeword chip set. By subtracting the synthesized post-cursor multipath echo from the received codeword, the input to an associated branch codeword correlator is effectively a xe2x80x9ccleaned-upxe2x80x9d version of the received codeword chip set from which multipath-based chip smearing has been removed. The output of each codeword correlator is coupled to a peak detector which selects the largest output as the transmitted codeword.
A more computationally efficient scheme for synthesizing the multipath channel impulse response may be realized by subtracting the respective FIR feedback filter tap stages from the received signal downstream of each codeword correlator. For reduced complexity the codeword correlator may be implemented as a fast Walsh (Hadamard) structure. Because the differential combining of the processing path for the received signal and the feedback taps is a post-correlation operation, there is no need to regenerate the feedback taps as each newly received codeword chip set is clocked into the correlator. This reduces implementation complexity by allowing the functionality of the synthesized tap path to be stored in a look-up table.